1. Field of the Invention
The present invention relates to a zinc dendrite inhibitor whereby in an electrolyte circulation-type secondary cell using zinc as an active material or in the case of electrochemical zinc plating, the zinc deposited and grown on an electrode surface is prevented from developing into dendritic crystals (hereinafter referred to as dendrites) and a smooth deposited zinc surface is produced.
2. Description of the Prior Art
The electrolyte circulation-type secondary cell is basically constructed as shown in FIG. 1 and it uses zinc halide-electrolytes. This cell comprises a unit cell 1 partitioned by a separating membrane (separator) 2 to form a positive electrode chamber 3 and a negative electrode chamber 4 on its sides, a positive electrode 5 positioned in the positive electrode chamber 3 and a negative electrode (zinc electrode) 6 positioned in the negative electrode chamber 4. A positive electrode electrolyte (e.g., ZnBr.sub.2 +Br.sub.2) is circulated through the positive electrode chamber 3 from a positive electrode electrolyte storage tank 7 by a pump 9 and a negative electrode electrolyte (e.g., ZnBr.sub.2) is circulated through the negative electrode chamber 4 from a negative electrolyte storage tank 8 by a pump 13. Numerals 11 and 12 designate valves which are opened during the charging and discharging, respectively.
When the above-mentioned cell is charged, bromine is deposited on the positive electrode side indicated by .sym. and zinc is deposited on the negative electrode side indicated by .crclbar.. While there is no problem if the deposited zinc forms uniformly and gradually increases its thickness on the negative electrode surface, generally the deposited zinc tends to develop into dendrites and grows locally due to various causes.
The process of formation of dendrites on the negative electrode surface is considered to approximately include the following steps. In other words, during the initial charging period metal zinc deposited on the negative electrode surface does not necessarily grow uniformly over the whole surface of the electrode but the metal zinc is deposited in a spotty manner. During this period, what can be considered as nuclei of dendrites are already produced. As a result, if the charging and discharging are continued in such a condition, the electrodeposition preferentially takes place at the spottily produced nuclei thus eventually producing dendrites.
When the dendrites grow on the negative electrode surface, the electric field concentrates so that the rate of growth of the dendrites increases and the chance of the zinc electrode surface contacting a fresh supply of the electrolyte decreases, thus not only deteriorating the cell efficiency but also causing the extremely fragile dendrites to fall off the electrode by a relatively small stress. As a result, the dendrites fallen off in this way cause clogging of the electrolyte circulation pipe and deterioration of the pump efficiency.
Assuming that such dendrites do not fall off the electrode but continue to grow so that the dendrites continue to grow progressively as shown at a, b and c in FIG. 2 or the growth progresses extremely, the dendrites directly contact with the positive electrode surface so that a electric short-circuit is formed and eventually the cell is damaged. Thus, the inhibition of growth of dendrites constitutes one of the very important requirements.
Such undesired phenomena occur even in the case of a electrochemical plating involving the electrolysis of zinc giving rise to a disadvantage of damaging the smoothness of a processed object.
In view of these requirements, various studies have been made on dendrite inhibitors and ionic or nonionic surface active agents, zinc electrochemical plating brighteners, etc., have for example been used. However, these agents are inadequate and leave much room for improvement with respect to such chemical resistances as resistance to bromine, the smoothness of deposited zinc with a large current density and large electrical quantity and so on. Also, these agents are not capable of maintaining stable performance during cyclic use over a long period of time and they are also inadequate in terms of the essential dendrite inhibitory effect.